Silicon photo diodes have excellent inherent linearity of current response to illumination. In addition, their rapid response, wavelength suitability and capability of being integrated into bipolar silicon integrated circuits have lead to their widespread use in automatic camera control circuits. This application imposes severe requirements on the photo diode. The illumination, which ranges from full sunlight to moonlight (or even less), involves about six orders of magnitude. Accordingly, if the bright response involves microampere currents, the low levels produce picoampere currents. This value is so small as to be masked by silicon junction leakage currents. It is also desirable that the response be rapid enough to respond to widely used flash illumination devices.
One solution to the leakage problem is to increase photo diode area. However, any increase in junction area increases perimeter which adversely affects leakage. In addition, large junction areas increase capacitance and thus can adversely affect associated circuit response time. Furthermore, such an approach results in increasing integrated circuit chip area. A more appropriate solution has been to operate the photo diode in a circuit that produces zero bias across the diode terminals. This greatly reduces leakage current and extends the linear response range. By using suitable circuitry, the response time can be made suitably small. However, it is difficult to devise a circuit that maintains zero bias in a manufacturable integrated circuit that meets all of the other performance requirements.